The difference between a proposition and its contents
I find that many people get confused in this area- the difference between a proposition and its contents. This confusion usually comes in the form of "if you're a strong atheist, doesn't that mean you have to be certain about the non-existence of God?", accompanied by a statement that strong atheism is dogmatic and therefore must be rejected.
Let's start with an easy example:
P1: I fervently believe that God exists.
Well, I think I'm pretty confident about that proposition, since it's part of my mental makeup and that's pretty accessible to me. The proposition itself states that God exists, with no qualification, so it's a strong proposition. We can rephrase both judgments like this:
P2: I am 100% confident in the belief that God exists.
This is trivial since there is no confusion in P1. But now look at this example:
P3: I am not so confident that God exists.
Now there is a difference. We have a difference between the belief and the person's confidence in it. This could be a "crisis of faith" of some sort. Either way, we can rephrase P3 as such:
P4: I am 50% confident that God exists.
Is this incoherent? Not at all. Even though "God exists" is a categorical statement, one does not need to believe it fully. Someone can have a crisis of faith about a very categorical statement of this kind. Does that mean no one has a crisis of faith, ever? Of course not.
So the following proposition must also be accepted:
P5: I am 50% confident that God does not exist.
Propositions such as P4 and P5 need not be crises of faith. They can also be the expression of a lack of information, or a desire not to explore the question too much. Now take a statement of agnosticism such as:
P6: I am fully confident that we should have no confidence whatsoever about the existence or non-existence of God.
Confusing? Once again remember the difference between the proposition and our confidence in it. In this case, our proposition is "we should have no confidence whatsoever about the existence or non-existence of God". The person is expressing full confidence in that proposition. He is quite certain that no one knows what they're talking about.
So there is no contradiction here. In all cases, there is a clear distinction between "what a person believes" and "how confident he is about it". One can hold categorical beliefs about which one is uncertain, and one can hold uncertain beliefs about which one is categorical.
Strong atheism, therefore, is not inherently dogmatic. It is possible to be certain, almost certain, uncertain, or lukewarm about the propositions "God does not exist" or "gods do not exist". But as long as you say them, you are a strong atheist. That's all that it means to believe in something.